Heat oil in heavy 6-quart saucepan over medium-high heat. Add onion, celery, bell pepper, and garlic and cook, stirring, until the vegetables begin to soften, 3-5 minutes. Add red pepper flakes, bay leaf, and black beans and stir well to combine the vegetables. Stir in the broth and water and season with salt and pepper to taste. Raise the heat to high and bring to a boil. Reduce the heat to medium-low, partially cover the saucepan, and simmer until the soup is thick, about 30 minutes.
Transfer the soup in batches to a food processor or blender or use an immersion blender, and puree until the soup is smooth and turns a mauve-gray color. Return the soup to the saucepan and reheat before serving.